The US-led coalition fighting Islamic State (IS) group jihadists in Iraq and Syria is better prepared to retake the Iraqi city Mosul than Syria's Raqa, a US military spokesman said Thursday.

Iraq's second city Mosul and Raqa, the IS group's de facto Syrian capital, are the coalition's top objectives.

"The plan to liberate Raqa is not as developed as the plan to liberate Mosul," coalition spokesman Colonel Steve Warren said during a video news conference broadcast from Baghdad.

The coalition can rely on the Iraqi Army to help conduct operations in Mosul, he added. "In Syria, we don't have that."

IS group fighters seized Mosul in June 2014 as they overran vast regions in northern and north-central Iraq, as well as in Syria.

The city holds special significance for the Islamic State group as the location where its leader Abu Bakr al-Baghdadi proclaimed his "caliphate" straddling Iraq and Syria.

In Syria, the coalition has only a small number of advisers on the ground working with the Syrian Democratic Forces (SDF), "essentially an irregular army" dominated by Kurdish militias, Warren said.

"(We) work with the leadership we have identified within the SDF to try develop a plan" to retake Raqa, he said. "That's ongoing, it's in the early stages, it's a continuing process."

The SDF numbers in the "tens of thousands," although that figure fluctuates, Warren added, saying the group includes around 5,000 Arab fighters.

The IS group has experienced setbacks on several fronts in Syria in recent weeks, including the ancient city of Palmyra, which the Russian-backed Syrian military retook late last month.

The jihadists also recently lost their main crossing point into Turkey, the town of al-Rai in Aleppo province, to factions fighting under the banner of the Free Syrian Army, the British-based Syrian Observatory for Human Rights said.

The town is one of at least 18 in Aleppo the IS group has lost after holding them for two years.